It has been previously proposed to provide a semi-conductor power device assembly in which the power device is thermally coupled to but electrically insulated from a base material by providing an alumina tile between the device and the base material, the alumina tile being metallised on one or both sides so as to enable it to be soldered to a heat spreader and being secured at its other side to the base material. The power device is soldered to the heat spreader. In order to ensure that there is a good thermal conduction between the power device and the base material, which is used as a heat skin, it is necessary for the solid alumina tile to be as thin as possible. It will be appreciated that the assembly process is relatively expensive and complicated because two soldering operations are required in addition to the securing of the assembly of power device, heat spreader and tile to the base material. Additionally, the tile is relatively expensive and is relatively fragile, thus further increasing costs and leading to handling problems in production.